Oligoclase Archives - Cochise College Geology

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Chemical Group: Silicate (plagioclase feldspar)

Chemical Formula: Oligoclase is a mid-composition plagioclase feldspar. Oligoclase ranges from 90% to 70% NaAlSi₃O₈and 70% to 50%CaAl₂Si₂O₈.

Color: White, gray, reddish, greenish, yellowish, brown, colorless. Sometimes displays schiller, a bluish or multi-colored opalescence.

Transparency: Transparent to translucent

Streak: Uncolored | Luster: Vitreous, pearly | Tenacity: Brittle

Hardness: 5 to 6 | Specific Gravity: 2.60 to 2.75

Fracture: Uneven to conchoidal

Cleavage: (001) perfect and (010) slightly less

Crystal Forms: Triclinic system. Crystals prismatic. Commonly shows polysynthetic (plagioclase) twinning.

Mineral habits: Massive, granular, coarse lamellar.

Mineral Associations: Diorite, gabbro, norite, andesite

Identifying Characteristics: Plagioclase twinning and schiller

Uses: Gemstone, decorator rock

Occurrences: Norway, Sweden; new York, Pennsylvania, North Carolina

Toxicity: When-swallowed: Moderate | When inhaled: High

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